Joint Federal Interagency Sedimentation Conferenet and Federal Interagency Hydrologic Modeling Conference 2015

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Using Oblique Digital Photography for Alluvial Sandbar Monitoring and Low-Cost Change Detection

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The maintenance of alluvial sandbars is a longstanding management interest along the Colorado River in Grand Canyon. Resource managers are interested both in the long-term trend in sandbar condition and the short-term response to management actions, such as controlled floods. Long-term monitoring is accomplished by a combination of annual topographic survey and quadrennial remote sensing. We present preliminary results for daily changes in sandbar topography based on the use of remotely deployed digital cameras. Starting in 2008, higher-resolution digital cameras capable of taking multiple photos daily were installed at many of the long-term monitoring sites, enabling rapid and low-cost analyses of sandbar response to geomorphic events. Controlled flood experiments have been periodically conducted to rebuild subaerial sandbars along the river, and the camera imagery is playing an increasing role in the evaluation of the floods’ effects. Post-flood images are compared to pre-flood images taken at the same discharge and analyzed for change in sandbar size by visual inspection. For a 2008 flood, 22 sites had topographic surveys corresponding with the photos. The visual analysis agreed with the topographic survey change analysis at 78% of the sites. A 2012 flood was evaluated foremost by photo analysis, and a smaller subset of surveyed sites was in 100% agreement with the outcomes. While this method of photo analysis is largely qualitative, it has been demonstrated to provide a rapid and fairly accurate metric for assessing the immediate downstream effects of controlled floods on bars.

Photos from the large  archive of imagery are now being orthorectified using a network of surveyed points at each site, in order to provide a more accurate assessment of how sandbar areas are changing in time. In addition, efforts are underway to extract 3D topographic data from future imagery, thus potentially providing a means to estimate sandbar volumes. This process involves the extraction of georeferenced shorelines from the orthorectified photos. These shorelines are used as contour lines of equal elevation based on the stage of the water surface. A pressure transducer deployed at the site provides the river stage, and thus elevation of the shoreline at the time the photo was taken. Grouping several of these extracted shorelines over a diurnal stage fluctuation will constitute a contour map of the wetted portion of the bar. Change in pre- and post-event digital elevation models derived from the contour sets will allow estimates of volumetric changes to be made, and provide quantifiable topographic data that can be processed quickly and at a low cost. Such data provide valuable insight into sandbar changes at times between ongoing annual topographic surveys.


Robert Tusso    
Grand Canyon Monitoring and Research Center

Daniel Buscombe    
Grand Canyon Monitoring and Research Center

Paul Grams    
Grand Canyon Monitoring and Research Center


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